1. Field of the Invention
The present invention relates to a manuscript document detecting system and method for use in a copying machine and/or a scanner, in particular, to a manuscript document detecting system and method for detecting a size and position of a manuscript document.
2. Discussion of Background
In recent years, various manuscript document detecting systems have been developed for detecting a size and position of a manuscript document in copying machines and/or scanners. Japanese Laid-open Patent Publication No. 2-308236 (1990) discloses a manuscript document detecting system capable of detecting a manuscript document size and position. In the disclosed system, when the closing of a device used for holding down a manuscript (e.g., a pressing plate) is detected, scanning is started, and a light-receiving signal from a light sensor is compared with a standard value. In this way, a determination is made of a presence or absence of the manuscript document. In such structure, the size and the positional shift of the manuscript document can be detected from scanning data at a time point when the state of the manuscript document changes from a state of being present to state of being absent and vice versa.
In another manuscript document detecting system, the manuscript document is scanned by radiating light toward the manuscript document from a lower portion of a document mounting stand. Light reflected from the manuscript document returning to an initial position is detected. In such an system, a standard signal representing a start of scanning light is provided and at that time counting of an internal clock is started. In this way, a measurement value representing a position of the scanning light is obtained. At a time of a first scanning, values of the measured scanning light position when first detecting the reflected light from the manuscript document and when finally detecting the reflected light from the manuscript document are processed as the data representing end positions of the manuscript document. In this way, the size and position of the manuscript document can be detected.
FIG. 8 is a graph for illustrating an example of an influence due to rotational unevenness of a motor used for scanning a manuscript document. In a method of optically scanning a manuscript document, it is generally required that a part of the optical system is rotated. However, it may be impossible to avoid errors due to rotational unevenness of the motor. For instance, as mentioned in the published specification of Japanese Laid-open Patent Publication No. 2-308236 (1990), if an element, such as an encoder, functioning mechanically in cooperation with the scanning system, is employed for measuring the position of the scanning light, although the rotational unevenness of the motor does not cause any problem theoretically, it is necessary to prepare a high-precision rotation detecting unit in order to obtain sufficient resolution for detecting the position and size of the manuscript document. Consequently, the manufacturing cost and complexity of the system are increased.
On the other hand, in a case where a measurement of scanning light position is performed by use of a clock in synchronism with the rotation of the motor, the cost may be decreased. However, a rotational unevenness of the motor directly exerts an influence upon such an system, and results in problems of precision. As shown in FIG. 8, the error due to the influence of the rotational unevenness of the motor is integrated (i.e., added up) and thereby increases as the scanning light is further from a starting position of scanning.
As previously discussed, a manuscript document detecting system has been developed wherein light is radiated toward a manuscript document and reflected light from the manuscript document is used to determine a size and position of the manuscript document. In Japanese Laid-open Patent Publication No. 2-308236 (1990), a manuscript document size and position detecting system is disclosed, wherein a closing of a device for holding down the manuscript document (e.g., a pressing plate) triggers a start of detection/scanning and a light receiving signal from a light sensor is compared with a standard value. In this way, a presence or absence of the manuscript document is determined so that the size and positional shift of the manuscript document is detected.
However, in such manuscript document detecting systems, since the reflected light includes light reflected not only from the manuscript document, but also may include other reflected light, such as from a device for holding down the document (e.g., pressing plate or ADF), body parts of an operator, etc., it may be necessary to distinguish light reflected from the manuscript document and from other sources in order to perform a precise detection of the size and positional shift of the manuscript document.
A method of detecting a manuscript document by detecting a distance utilizing a method of astigmatism may also be employed. Specifically, a focal point is set at a place optically more distant than a manuscript document mounting stand in view of the system main body. For instance, as shown in FIG. 12, light reflected from a manuscript document closer than the focal point is output as a positive voltage, while the light reflected from, for example, a pressing plate more distant from the focal point is output as a negative voltage. Only the positive voltage is detected as a binarized output signal RCV of the output light receiving signal PDO. In this figure, signal T9 represents a driving period for driving a light emitting source.
However, in such systems, as shown in FIG. 13, when light reflected from the pressing plate increases due to, for example, external disturbance light and/or the angle of the incident light, even though the scanning light is modulated and driven with a predetermined period, the disturbance light and the scanning light cannot be distinguished from each other. In this case, the duty cycle of the modulation/drive signal T9 is 50% for both the disturbance light and the light reflected from the manuscript document, as shown in FIG. 13, and the reflected light due to the disturbance light is improperly detected as light reflected from the manuscript document, resulting in manuscript document detection errors.
Furthermore, in systems which employ a device, such as an encoder, for detecting rotation of a motor used for driving an optical scanning system, influence exerted by rotational unevenness of the motor are not compensated, resulting in a system with reduced precision.